The effect of carbon nanotube properties on the degree of dispersion and reinforcement of high density polyethylene Melanie Morcom a , Ken Atkinson b , George P. Simon a, * a Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia b CSIRO Materials Science and Engineering, Australia article info Article history: Received 21 July 2009 Received in revised form 2 March 2010 Accepted 26 April 2010 Available online 15 May 2010 Keywords: Nanotubes Composites Polyethylene abstract The efficacy with which a range of nanotubes could reinforce a high density polyethylene (HDPE) matrix was investigated, in relation to nanotube diameter, purity, functionalization, alignment and nanotube bulk density. Composites were prepared by melt blending multiwall carbon nanotubes (MWNTs) with high density polyethylene (HDPE), followed by the injection molding of tensile specimens. At a 5 wt% loading, the most effective nanotubes were those of large diameter, received in an aligned form with low bulk density, producing a 66% increase in elastic modulus and a 69% improvement in yield stress. This was contradictory to theoretical mechanics calculations that predicted an increasing degree of rein- forcement for nanotubes of reduced diameter. This difference was explained by the higher degree of dispersion observed in the composites with MWNTs of greater diameter. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Theoretical and modeling studies, such as those by Thostenson and Chou [1], show that smaller diameter nanotubes have a higher potential for reinforcement due to their higher effective elastic modulus and strength. A review of the literature indicates, however, relatively few reports on the comparative reinforcing abilities of different nanotubes in the same matrix, and these have all been in non-olefinic based materials. Gojny et al. [2] investigated the reinforcement of epoxy with various nanotubes, comparing single walled carbon nanotubes (SWNTs), double walled carbon nanotubes (DWNTs) and MWNTs, as well as amino-functionalized DWNTs and MWNTs. At the concentration of 0.1 wt%, SWNTs provided the largest increase in tensile strength and DWNTs the greatest improvement in elastic modulus. At higher concentrations (up to 0.5 wt%), amino- functionalized DWNTs showed the largest increase in both elastic modulus and strength. Since the reinforcing potential of SWNTs is higher due to their higher values of strength and elastic modulus, it is likely that the poorer properties of the SWNTeepoxy composites at concentrations greater than 0.1 wt% were due to a lack of dispersion. Likewise, the amino-functionalization of the DWNTs was thought to bring about a higher degree of dispersion at the higher concentrations (and thus an increase in elastic modulus) by increasing the surface polarity of the carbon nanotubes that inter- acted with the matrix. Cadek et al. [3] studied the reinforcement effects of six different types of carbon nanotubes on freestanding poly(vinyl alcohol) films prepared by solution mixing at concentrations of 0.005 vol fraction (w1 wt%). The low diameter MWNTs gave the highest level of reinforcement, the level of reinforcement being inversely propor- tional to nanotube diameter. The polymer was found to crystallize on the surface of the nanotubes, thus leading to a greater degree of reinforcement due to the higher modulus of this induced crystal- line phase. The coating was the same thickness for each type of nanotube and thus the degree of crystallinity was linearly depen- dent on nanotube concentration and inversely proportional to the diameter of the nanotubes. The higher reinforcement provided by the finer MWNTs in their work may thus have been due to the increased crystallinity of the composite rather than being directly due to higher stiffness and strength imparted by the nanotubes. Potschke et al. [4] investigated the effect of MWNT diameter on both the dispersion and the mechanical properties of poly- carbonate nanocomposites. By adjusting the temperature at which the MWNTs were synthesized, MWNTs with mean diameters of 22 and 28 nm were produced while maintaining lengths of up to 10 mm in both samples. Larger diameter MWNTs were found to be better dispersed in the polycarbonate by melt mixing than the thinner nanotubes, however there was no observable effect of nanotube diameter on the stressestrain curves of composites. The work reported in this paper explores the extent to which the nanotube diameter has an effect on the effective reinforcement of * Corresponding author. Tel.: þ61 3 9905 4936; fax: þ61 3 9905 4934. E-mail address: george.simon@eng.monash.edu.au (G.P. Simon). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.04.053 Polymer 51 (2010) 3540e3550